1. Field of the Invention
This invention relates to a method of calculating a work amount parameter, such as indicated mean effective pressure, which is indicative of the amount of work done by an internal combustion engine.
2. Description of the Related Art
Conventionally, a method of calculating indicated mean effective pressure has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H11-182319. In this method, the indicated mean effective pressure is calculated as follows: In-cylinder pressure, i.e. pressure in a combustion chamber of an internal combustion engine, is detected by an in-cylinder pressure sensor of a piezoelectric element type which is mounted to a spark plug of the engine. Then, the indicated mean effective pressure is calculated by subtracting a value obtained by integrating the product of the detected in-cylinder pressure and the rate of change in volume of the combustion chamber (hereinafter simply referred to as “the volume change rate”) as the piston of the engine is moved from a bottom dead center (BDC) position to a top dead center (TDC) position, from a value obtained by integrating the product as the piston is moved from the TDC position to the BDC position.
In the above conventional method, however, there can occur a large error in the calculated indicated mean effective pressure. More specifically, an in-cylinder pressure signal indicative of the detected pressure in the combustion chamber delivered from the in-cylinder pressure sensor, as described above, usually contains noises, and hence if in-cylinder pressure determined based on the in-cylinder pressure signal is used as it is for calculation of the indicated mean effective pressure, there is a fear that the calculated indicated mean effective pressure contains a large error due to influence of the noises.
To avoid the above inconvenience, it is envisaged, for example, to eliminate noises contained in the in-cylinder pressure data by filtering the in-cylinder pressure signal by a low-pass filter having a relatively low cutoff frequency. In this case, however, the low-pass filter generally has a characteristic that it delays the signal as it filters the same, and the delay is increased as the cutoff frequency of the low-pass filter is lower. This causes a relatively large phase delay of the filtered in-cylinder pressure with respect to the actual in-cylinder pressure. In this connection, since the indicated mean effective pressure is calculated based on the product of the in-cylinder pressure and the volume change rate, it is possible to regard the indicated mean effective pressure as a correlation coefficient between the in-cylinder pressure and the volume change rate. Further, a correlation coefficient between two data tends to be largely changed as phase deviation of one of the data from the phase of the other is larger. Therefore, due to the large phase delay which the in-cylinder pressure experiences as described above, the calculated indicated mean effective pressure comes to suffer from a significant error or difference from the proper value.